Several new enzymes involved in the synthesis, modification, and degradation of DNA are found soon after infection of Escherichia coli by T4 bacteriophage. We are trying to isolate and characterize the products of several phage genes which are known to be required for DNA synthesis whose enzymatic functions are unknown, including those encoded by the phage genes 44-62, 45, and 41. We are studying the mechanism of the reactions catalyzed by the T4 DNA polymerase in vitro and are comparing the properties of wild-type and mutant T4 DNA polymerases in order to explain the observed increase or decrease in mutation frequency associated with particular mutations in the structural gene (gene 43) for the T4 DNA polymerase. We have developed a new method for the identification of bacteria with a mutator phenotype, which we will use to search for such mutants of E. coli.